1. Field of the Invention
The present invention relates to a surface mount type crystal oscillator provided with adjustment terminals on an outer surface of a container, and more particularly, to a surface mount type crystal oscillator meeting downsizing requirements.
2. Description of the Related Arts
For its small size and lightweight, a surface mount type crystal oscillator which houses a quartz crystal unit and an oscillation circuit using this crystal unit in a surface mount type container is incorporated, for example, in a portable electronic apparatus as a reference source for a frequency and time. One of such surface mount type crystal oscillators is provided with, on an outer surface of the container, adjustment terminals including a writing terminal to write temperature compensation data into the crystal oscillator and an inspection terminal used to inspect characteristics of the crystal unit. However, a crystal oscillator has been increasingly downsized in recent years and it is becoming increasingly difficult to provide these adjustment terminals on the outside surface of the container of the surface mount type crystal oscillator.
FIG. 1A is a sectional view showing an example of the configuration of a conventional surface mount type crystal oscillator, FIG. 1B is a front view of the crystal oscillator and FIG. 1C is a bottom view of the crystal oscillator. FIG. 1A shows a section of FIG. 1C along line A-A.
The illustrated surface mount type crystal oscillator is provided with container body 1 which is made of laminated ceramic and has a recess, IC (Integrated Circuit) chip 2 housed in the recess of container body 1, and quartz crystal blank 3. A metal ring made of a metal thick film is provided on the periphery of the opening of the recess of container body 1. IC chip 2 and crystal blank 3 are hermetically sealed in the recess by covering and closing the recess with metal cover 4 by seam-welding metal cover 4 with the metal ring. The crystal unit is configured especially by hermetically sealing crystal blank 3 in container body 1.
Container body 1 has a flat, substantially rectangular parallelepiped outside shape having short sides and long sides when mounted on a wiring board and viewed from above, and a step portion is formed in an inner wall of container body 1. Mounting electrodes 5 are formed in the four corners of the outer bottom face of container body 1 to surface-mount this crystal oscillator on the wiring board. Each mounting electrode 5 is formed so as to extend not only over the outer bottom surface but also from the outer bottom surface over the side surface along the long side of container body 1. A portion of mounting electrode 5 formed over the side surface of container body 1 is called “end face electrode 5a.”
In the illustrated crystal oscillator, the laminated ceramic which makes up container body 1 has a four-layer structure of first layer A, second layer B, third layer C and fourth layer D in that order from the bottom surface side. End face electrodes 5a are formed on the end faces of first layer A to third layer C but are not provided at positions corresponding to the end faces of fourth layer D.
Furthermore, writing terminals 6a for writing temperature compensation data and characteristic inspection terminals 6b used for a characteristic inspection of the crystal unit are provided on the outer side surface of container body 1 as adjustment terminals. Two writing terminals 6a are provided on each side surface along the long side of container body 1. At this time, end face electrodes 5a are formed on the side surfaces along the long sides of container body 1 at both ends thereof as described above and writing terminals 6a are interposed between these end face electrodes 5a. Furthermore, each of characteristic inspection terminals 6b is provided substantially at the center of each side surface along the short side of container body 1. These writing terminals 6a and characteristic inspection terminals 6b are formed on the end faces of second layer B and third layer C out of the four layers of the laminate ceramic and not formed on the end faces of first layer A and fourth layer D.
It should be noted that the reason that end face electrodes 5a and adjustment terminals 6a, 6b are not formed on the end faces of fourth layer D is to prevent electric short circuits between these electrodes, terminals and metal ring. Furthermore, the reason that adjustment terminals 6a, 6b are not formed on the end faces of first layer A either is to prevent electric short circuits between the adjustment terminals and wiring board when the crystal oscillator is mounted on the wiring board.
The method of forming such end face electrodes 5a and adjustment terminals (i.e., writing terminals 6a and characteristic inspection terminals 6b) will be explained. When forming the container body for surface mounting made of laminated ceramic, it is a general practice to use ceramic green sheets each having a size corresponding to a plurality of container bodies, laminate and burn the sheets, and then divide the burned sheets into individual container bodies. Therefore, a ceramic sheet in a size corresponding to a plurality of container bodies 1 is also used here as the ceramic sheet of each layer. When container body 1 is formed of laminated ceramic, end face electrodes 5a and adjustment terminals 6a, 6b are formed by so-called through-hole work after forming an electrode pattern of W (tungsten) or the like in the ceramic sheet of each layer by printing and uniting the ceramic sheets of the respective layers. In the illustrated example, second layer B and third layer C are united first, adjustment terminals 6a, 6b are formed by through-hole work, and then first layer A is united and end face electrodes 5a are formed by through-hole work. Fourth layer D is united and then all these layers are burned, and after forming, for example, gold plating on the electrode pattern, the united ceramic sheet is then divided into individual container bodies 1. Container body 1 provided with end face electrodes and adjustment terminals is formed in this way. When through-hole work is performed, through holes are also provided in first layer A and fourth layer D to make through-hole surfaces thereon even when no electrode pattern is formed on first layer A and fourth layer D.
IC chip 2 has a substantially rectangular shape and is configured to integrate an oscillation circuit using crystal blank 3 and a temperature compensation mechanism for compensating the frequency temperature characteristic of crystal blank 3 in a semiconductor substrate. The oscillation circuit and temperature compensation mechanism are formed on one main surface of the semiconductor substrate through a normal semiconductor device fabricating process. Therefore, suppose the surface of the semiconductor substrate on which the oscillation circuit and temperature compensation mechanism are formed out of both main surfaces of IC chip 2 is called a “circuit forming surface.” A plurality of terminals for connecting IC chip 2 to an external circuit are also formed on the circuit forming surface. These terminals include a power supply terminal, a grounding terminal, an oscillation output terminal, a pair of connection terminals for connection with the crystal blank, an AFC (automatic frequency control) terminal to which an AFC signal is supplied and input terminals for writing data into the temperature compensation mechanism or the like.
A circuit pattern (not shown) made up of circuit terminals and conductive paths corresponding to the terminals on the IC chip 2 side is formed on the bottom surface of the recess of container body 1. By joining the terminals of IC chip 2 and circuit terminals on the bottom surface of the recess through ultrasonic thermal compression bonding using, for example, bumps 7, IC chip 2 is secured to the bottom surface of the recess of container body 1 with the circuit forming surface facing the bottom surface of the recess. The power supply terminal, grounding terminal, output terminal and AFC terminal out of the terminals of IC chip 2 are connected to corresponding end face electrodes 5a through conductive paths formed on the lamination surface of the laminated ceramic and thereby connected to mounting electrodes 5, too. Furthermore, a pair of connection terminals for connection with crystal blank 3 are also connected to characteristic inspection terminals 6b through similar conductive paths and the input terminals are also connected to writing terminals 6a. 
Crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank provided with excitation electrodes (not shown) on both main surfaces thereof and extending electrodes extend from these excitation electrodes toward both sides of one end of crystal blank 3, respectively. Crystal blank 3 is held level in the recess as illustrated by fixing the extending electrodes to the top face of the step portion in the recess of container body 1 using conductive adhesive 8. Conductive paths (not shown) are also formed in the step portion of the recess, and crystal blank 3 is electrically connected to the connection terminals of IC chip 2 through the conductive paths and thereby electrically inserted into an oscillation closed loop of the oscillation circuit in IC chip 2. Since the connection terminals of IC chip 2 are also connected to characteristic inspection terminals 6b as described above, crystal blank 3 is connected parallel to the oscillation circuit and the pair of characteristic inspection terminals 6b. 
With such a crystal oscillator, it is possible to operate the oscillation circuit, measure an oscillating frequency and adjust the oscillating frequency using mass load on crystal blank 3 by causing a probe of a measuring instrument to contact end face electrodes 5a in a condition in which IC chip 2 and crystal blank 3 are provided whereas no metal cover 4 is provided. Furthermore, after sealing IC chip 2 and crystal blank 3 in the recess with metal cover 4, it is possible to cause the probe to contact writing terminal 6a and write temperature compensation data into the temperature compensation mechanism in IC chip 2. Furthermore, by causing the probe to contact characteristic inspection terminal 6b, it is possible to measure the vibration characteristic of crystal blank 3 as a crystal unit alone. In this way, since end face electrodes 5a, writing terminals 6a, characteristic inspection terminals 6b are exposed on the side surfaces of container body 1, it is possible to perform various inspections and adjustment work by causing the probe to contact the electrodes and terminals from the lateral directions without mounting the crystal oscillator on the wiring board. In the case of a crystal oscillator requiring no temperature compensation data to be written, writing terminals 6a out of the adjustment terminals need not be provided.
As for devices for surface mounting, mounting electrodes formed on the bottom surface of the container body are also extended to a certain degree over the side surfaces of container body 1 to form end face electrodes and solder fillet is thereby formed on each of the side surfaces when the device is mounted on the wiring board. Since the quality of soldering can be accurately judged according to the presence/absence of the solder fillet, it is general to provide end face electrodes in a surface mount type device. The above described crystal oscillator is designed such that the probe can contact the end face electrode by forming such a large end face electrode.
As for the above described crystal oscillator, through-hole work is used for the laminated ceramic when forming end face electrodes 5a and adjustment terminals 6a, 6b, and therefore end face electrodes 5a and adjustment terminals 6a, 6b are formed into a concave shape, which makes contact by the probe easier.
However, due to further downsizing of the surface mount type crystal oscillator in the above described structure, when the planar outside size thereof is, for example, 2.5×2.0 mm or smaller, adjustment terminals 6a, 6b come to contact end face electrodes 5a, and therefore it is difficult to provide such adjustment terminals. Especially when writing terminals 6a are provided for a temperature compensation type crystal oscillator, the number of adjustment terminals increases together with characteristic inspection terminals 6b for the crystal unit, and so the problem becomes more serious in particular. Writing terminals 6a and characteristic inspection terminals 6b need to be as large as 0.4×0.4 mm or more to keep contact with the probe of the measuring instrument.